Numerous pads and cushions have been developed to address the needs of human health, comfort and safety for sitting or reclining, and for other load supporting purposes. Many of these pads provide other features as well such as insulating the load from the temperature environment beneath the pad.
The problem is that most of the pads and cushions in the art absorb energy relatively uniformly, both over the surface of the pad, and as a function of depression of the pad as it absorbs and supports the load. Thus a pad supporting an anatomical load of a certain weight and shape supports that load in the same way everywhere at once, and the "boney" parts of the human anatomy, especially the ischia, either "bottom out" through the pad, or compress the pad completely, to the relatively harder and less yielding surface beneath, or at best receive no better support than the rest of the anatomy. Pressure studies conducted at Queen Alexander Hospital in Victoria, British Columbia, Canada have shown that the ischia can exert as much as 10 to 15 times the point load pressure on a support surface as the pressure exerted by softer or more spread out structures of the anatomy.
As a corollary proposition therefore, given a support surface of even density and resiliency characteristics, these greater pressure zones will, in effect, experience greater penetration into the surface. Where the surface is otherwise either incapable of sufficient resiliency to absorb the entire point load, or possessed of insufficient thickness so that the load "bottoms out" before full load absorption is achieved, then the residual pressure is instead absorbed by the respective portion of the anatomy. Thus in situations where there musk be long term support, such as for a secretary type chair cushion or a wheel chair to anatomical stress and overuse injuries. This commonplace phenomenon can give rise to pressure sores, particularly in persons who have to spend years at a time in a fixed position, such as wheelchair bound persons, or persons confined to bed. It is known that the primary cause of pressure sores is extended high pressure on tissue causing a lack of blood supply and associated tissue breakdown Known contributing causes further include temperature rise which increases metabolism to already blood starved tissue, and moisture at the skin surface which reduces the tensile strength of otherwise healthy tissue and makes it and the associated tissue even more prone to breakdown.
It is well known that unrelieved high pressure will give rise to sores and eventually to even more serious deep tissue breakdown, and complications thereof, which have been known to go so far as to result in death. In fact, a paper entitled "The Availability And Utilization Of Assistive Technology To Meet The Seating And Mobility Needs Of Severely Disabled And Elderly Persons" published in 1989 by RESNA Press in Washington, D.C. reports that pressure sores have an incidence of one million per year in the severely disabled population, while the average cost for treating a severe pelvic pressure sore is estimated at $30,000. A 1990 RESNA paper reports that some 13,000,000 wheelchairs are in use in the United States, and of that population 6,000,000 could benefit from some form of special seating cushion. In North America, it is reported that 70,000 people die every year from complications associated with pressure sore related illnesses. The total estimated annual medical expense associated with this unfortunate phenomenon exceeds two billion dollars.
What is needed is a load bearing structure or pad that differentially supports such anatomical irregularities to prevent bottoming out while simultaneously providing comfort and freedom from such stress injuries. Such a pad must nonetheless support the entire anatomy in a design load range so that the whole anatomical load is distributed more uniformly over the whole pad, while having the deformation of the pad closely follow natural body contours so that point loads are adequately absorbed by the pad and not by the anatomy. Ideally, at design load, the pad would still have adequate depth under the anatomical projections to accommodate live load shock absorption.
In addition, conventional pad structures either do not provide for, or have inadequate provision for, ventilation and associated moisture sublimation and cooling, nor do they provide much in the way of prevention for sliding around on, and sliding off of, the pad.